SPOTLIGHTTHE ARROWS IN ORGANISM-SUBSTRATE INTERACTIONS

It really is a wonderful opportunity to be able to write a Spotlight piece as past Co-editor of PALAIOS. After all, since my time as a student in the deep, deep South (i.e., south of the Rio Grande), I loved reading the front piece of the freshly arrived-in-the-mail PALAIOS journal—sent by snail mail at that time and patiently awaited! Those lines, typically provocative, revealed their authors in a very different light, sharing personal perspectives, with the fresh flavor of ideas presented in an almost colloquial way, without the necessary rigor and structure of a more formal scientific contribution. These lines try to follow this spirit of a Spotlight article and hopefully will trigger some excitement and out-of-the box thinking in a student somewhere, maybe in a distant corner of our planet, or in a place close to home. Since Dolf Seilacher’s seminal work unravelling the links between environmental factors and benthos response, ichnology has become instrumental for facies analysis and paleoenvironmental reconstructions. The underlying reasoning was straightforward. Trace fossils can be seen as evidence of organisms’ behavior, and that behavior is strongly affected by environmental factors. Therefore, careful reading of the trace-fossil record provides valuable information that can be used in paleoenvironmental interpretations. Seilacher built up this approach with his work in rocks of different ages formed in a wide variety of environments, from strata close to his home town of Tübingen, such as the Jurassic of southern Germany, to localities visited during far away expeditions, most notably the Cambrian of the Salt Range in Pakistan (e.g., Seilacher 1955). According to this view of the ichnologic record, the fact that similar trace-fossil assemblages are present in specific sedimentary facies all through the geologic column reflects behavioral convergence: different types of animals employ similar responses to deal with similar sets of problems. This underlying reasoning is at the core of the ichnofacies model. If we unlock the behavioral signal recorded in trace fossils, we can unravel the role played by the different environmental factors (or at least identify dominant controlling factors). Accurate integration of the ichnologic dataset with sedimentologic and stratigraphic information allows a dynamic reconstruction of the environmental setting and provides an interpretation in terms of animal-substrate interactions, depositional processes, and sedimentary environments. Note that, contrary to a common misconception, trace fossils neither indicate depositional processes, nor sedimentary environments, but are a biological response to environmental factors (e.g., oxygen, energy, salinity). A bivalve living within the sediment produces an escape trace in response to being buried under an episodic sedimentation event. This unhappy bivalve cares not about the nuances of depositional dynamics, and whether their being buried is due to a storm, a tidal surge, a turbidity current, or a hyperpycnal flow: its top priority is survival and to avoid being buried deeper than its habitable zone. Sedimentary geologists and most ichnologists working in outcrops and cores from an applied perspective have essentially adopted this adaptationist and externalist view of the field for over half a century now. The environment dictates and the organisms react according to their life history traits (i.e., body size, locomotory capabilities, and style of bioturbation). From a pragmatic standpoint, this certainly has been a highly successful approach because it led to numerous refinements of a wide spectrum of facies models (e.g., Pemberton et al. 2001; MacEachern et al. 2009; Buatois and Mángano 2011), and the use of this approach remains valid. However, the downside of this perspective on animal-substrate interactions is that the activity of animals is perceived as completely subordinate to the processes and the setting. The explanatory arrow only goes one way—from the environment to the organisms: abiotic factors dictate the fate of the living world. This view of ichnology has gradually been transformed during the last few decades because of the convergence of two lines of thought or perspective. From an ecologic perspective, the concept of ecosystem engineering places pre-eminence to the way organisms modify, maintain, and create habitats, resulting in profound changes at the ecosystem scale (Jones et al. 1994; Visser et al. 2013). Bioturbation, an archetypal example of allogenic engineering, can be seen in this light (Meysman et al. 2006). This perspective can be traced back to Darwin’s (1881) study on the effects of bioturbation on soil formation as the result of earthworm activity (Pemberton and Frey 1990). In marine environments, the importance of biogenic reworking is illustrated by the fact that the disappearance of key bioturbators may severely affect ecosystem structure and functioning, resulting in sharp decreases in biodiversity (Lohrer et al. 2004; Solan et al. 2009). From a geobiologic perspective, it has long been known that the geosphere and the biosphere co-evolve, highlighting biological processes as a driving force in the evolution of our planet. The role of cyanobacteria in the oxygenation of the oceans 2.4–2.0 billion years ago during the Great Oxidation Event (GOE) represents a familiar example. From a perspective closer to the heart of ichnologists, the onset of effective bioturbation around the Ediacaran–Cambrian transition is a prime example of geobiologic feedbacks on a deep-time scale (Seilacher 1999). By looking at bioturbation and the trace-fossil record through the lens of organisms being active players on an ecologic stage, a modification of the explanatory arrow is required. From this perspective, organisms are no longer simply reacting (or succumbing) to the vicissitudes of stressing inorganic factors: they are actively modifying and actually creating their own environment. The role of bioturbating (and bioeroding) organisms can be explored at different scales, from those in response to a single burrow to the effect of infaunal burrowing communities on ecosystems and the whole biosphere. The seeds of this approach were planted 40 years ago with the innovative work of Robert Aller (1980), who modeled the impact of dwelling burrows on the distribution of pore-water solutes and their sediment-water fluxes. This research group and the so-called Nereis Park group (http://nereispark.org/) continue to produce exciting results in this